Polymer solar cells (PSCs) provide an avenue to inexpensive renewable energy by large scale printing of lightweight and flexible materials. PSCs typically comprise multiple layers, where efficient electronic communication at each interface is crucial for achieving high efficiency. As such, interfacial engineering is needed to enhance device performance. See, e.g., Yip, H.-L.; Jen, A. K.-Y. Energy Environ. Sci. 2012, 5, 5994; He, Z.; Wu, H.; Cao, Y. Adv. Mater. 2014, 26, 1006. For example, interlayers located between the active layer and conductive electrodes improve the selectivity of charge transport, and minimize series resistance, leading to PCE values exceeding 9% for single junction PSCs. See, e.g., He, Z.; Thong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 591; Yao, K.; Salvador, M.; Chueh, C.; Xin, X.; Xu, Y.; Dane, W.; Hu, T.; Chen, Y.; Ginger, D. S.; Jen, A. K. Adv. Energy Mater. 2014, 4, 1400206; Zhang, W.; Wu, Y.; Bao, Q.; Gao, F.; Fang, J. Adv. Energy Mater. 2014, 4, 1400359; Guo, X.; Zhang, M.; Ma, W.; Ye, L.; Zhang, S.; Liu, S.; Ade, H.; Huang, F.; Hou, J. Adv. Mater. 2014, 26, 4043; Li, C.-Z.; Chang, C.-Y.; Zang, Y.; Ju, H.-X.; Chueh, C.-C.; Liang, P.-W.; Cho, N.; Ginger, D. S.; Jen, A. K.-Y. Adv. Mater. 2014. doi: 10.1002/adma.201402276. For example, a blend of poly(ethylenedioxythiophene) and poly(styrene sulfonate) (PEDOT:PSS) functions as a solution processible hole-selective anode modification layer that has evolved into standard use in PSCs. Recent efforts have been devoted to developing new cathode modification layers to enhance electron extraction efficiency. Numerous organic small molecule interlayers have been integrated into PSCs with noteworthy device improvements, such as functional fullerenes and perylene-diimides and oligomeric fluorenes. See, e.g., Yao, K.; Salvador, M.; Chueh, C.; Xin, X.; Xu, Y.; Dane, W.; Hu, T.; Chen, Y.; Ginger, D. S.; Jen, A. K. Adv. Energy Mater. 2014, 4, 1400206; Page, Z. a.; Liu, Y.; Duzhko, V. V.; Russell, T. P.; Emrick, T. Science. 2014, 346, 441; O'Malley, K. M.; Li, C.-Z.; Yip, H.-L.; Jen, A. K.-Y. Adv. Energy Mater. 2012, 2, 82; Yang, X.; Chueh, C.; Li, C.; Yip, H.; Yin, P.; Chen, H.; Chen, W.; Jen, A. K. Adv. Energy Mater. 2013, 3, 666; Chueh, C.; Chien, S.; Yip, H.; Salinas, J. F.; Li, C.; Chen, K.; Chen, F.; Chen, W.; Jen, A. K. Adv. Energy Mater. 2013, 3, 417; Mei, Q.; Li, C.; Gong, X.; Lu, H.; Jin, E.; Du, C.; Lu, Z.; Jiang, L.; Meng, X.; Wang, C.; Bo, Z. ACS Appl. Mater. Interfaces 2013, 5, 8076; Li, S.; Lei, M.; Lv, M.; Watkins, S. E.; Tan, Z.; Zhu, J.; Hou, J.; Chen, X.; Li, Y. Adv. Energy Mater. 2013, 3, 1569; Lai, Y.-Y.; Shih, P.-I.; Li, Y.-P.; Tsai, C.-E.; Wu, J.-S.; Cheng, Y.-J.; Hsu, C.-S. ACS Appl. Mater. Interfaces 2013, 5, 5122; Li, X.; Zhang, W.; Wu, Y.; Min, C.; Fang, J. J. Mater. Chem. 2013, 1, 12413; Duan, C.; Zhong, C.; Liu, C.; Huang, F.; Cao, Y. Chem. Mater. 2012, 24, 1682; Zhang, Z.-G.; Qi, B.; Jin, Z.; Chi, D.; Qi, Z.; Li, Y.; Wang, J. Energy Environ. Sci. 2014, 7, 1966; Zhang, W.; Wu, Y.; Bao, Q.; Gao, F.; Fang, J. Adv. Energy Mater. 2014, 4, 1400359. Polymer interlayers provide advantages of both facile solution processing and robust film formation, with two recently reported examples being poly(ethyleneimine) (PEI) and tertiary-amine substituted polyfluorene (PFN). See, e.g., Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Brédas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B. Science. 2012, 336, 327; Woo, S.; Hyun Kim, W.; Kim, H.; Yi, Y.; Lyu, H.-K.; Kim, Y. Adv. Energy Mater. 2014, 4, 1301692; Gu, C.; Chen, Y.; Zhang, Z.; Xue, S.; Sun, S.; Zhong, C.; Zhang, H.; Lv, Y.; Li, F.; Huang, F.; Ma, Y. Adv. Energy Mater. 2014, 4, 1301771.
Cathode modification layers lead to negative interfacial dipoles (Δ) that lower the electrode work function and increase the electrostatic potential across the device. See, e.g., Worfolk, B. J.; Hauger, T. C.; Harris, K. D.; Rider, D. a.; Fordyce, J. a. M.; Beaupré, S.; Leclerc, M.; Buriak, J. M. Adv. Energy Mater. 2012, 2, 361. This enables the use of stable, high work function metals in devices, while the enhanced E-field increases free charge generation and extraction efficiency to maximize short-circuit current density (JSC) and fill factor (FF). The interfacial dipole moreover increases the anode-cathode work function offset (ΦA-C), thus enhancing open circuit voltage (VOC). See, e.g., He, Z.; Zhong, C.; Huang, X.; Wong, W.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, 23, 4636. Polar semiconducting polymers, such as conjugated polyelectrolytes (CPEs) and conjugated polymer zwitterions (CPZs), provide large negative Δ values, with an inherent tunability of electronic properties. See, e.g., Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 8416; Jo, J.; Pouliot, J.-R.; Wynands, D.; Collins, S. D.; Kim, J. Y.; Nguyen, T. L.; Woo, H. Y.; Sun, Y.; Leclerc, M.; Heeger, A. J. Adv. Mater. 2013, 25, 4783; Kang, R.; Oh, S.; Kim, D. ACS Appl. Mater. Interfaces 2014, 6, 6227; Liu, F.; Page, Z.; Duzhko, V.; Russell, T. P.; Emrick, T. Adv. Mater. 2013, 25, 6868; Duan, C.; Zhang, K.; Guan, X.; Zhong, C.; Xie, H.; Huang, F.; Chen, J.; Peng, J.; Cao, Y. Chem. Sci. 2013, 4, 1298; Page, Z. A.; Liu, F.; Russell, T. P.; Emrick, T. Chem. Sci. 2014, 5, 2368; Page, Z. A.; Liu, F.; Russell, T. P.; Emrick, T. J. Polym. Sci., Part A Polym. Chem. 2014. doi: 10.1002/pola.27349.
One shortcoming of current cathode modification layers is their inefficiency of electron transport, which limits their operational thickness to 5 nanometers (nm) or less. Such is the case for insulating and p-type polymers. To circumvent this, interlayers with appreciable electron transport properties are needed to reduce the potential for charge build-up and surface recombination. An ideal cathode modification layer would reduce electrode work function (Φ), have solubility orthogonal to that of the photoactive layer, exhibit good film forming properties (wettability/uniformity), transport electrons selectivity, possess large electron affinity (EA) and exhibit long-term stability. No current interlayers satisfy all of these requirements. Accordingly, there is a continuing need for new interlayer materials to overcome the above-described technical limitations.